Studies of the oral actinomycetes are continuing toward defining the adherence mechanisms of these bacteria to tissue surfaces. Investigations of the Actinomyces viscosus T14V cell surface have identified two distinct types of bacterial fimbriae (Agl and Ag2) that differ in their functional properties. The Ag2 fimbriae are the sites of a lactose-sensitive lectin activity that mediates coaggregation of actinomycete cells with certain plaque streptococci and the adherence of bacteria to mammalian cells. In contrast, those fimbriae designated as Ag1 or VA1 appear to play a critical role in bacterial adherence to saliva-treated beads of hydroxyapatite, an interaction that is unaffected by various sugars. The isolation and characterization of mutant bacterial strains lacking Agl fimbriae, Ag2 fimbriae or both components have provided additional support for the distinct functions of each structure. Morever, differences in the distribution of these fimbrial components on typical strains of A. viscosus and A. naeslundii appear to be correlated with certain well established differences in the adherence properties of these species. Thus, separate and specific structures on these bacteria seem to be involved in their adherence to different surfaces within.